The invention relates to vacuum brake boosters.
Vacuum brake boosters as such have been known for some time and millions of them are in use for boosting the actuating forces of a vehicle hydraulic brake system and hence keeping said forces at a comfortably low level for the driver of a vehicle.
Recently, there has been a growing tendency to equip vacuum brake boosters with so-called brake assist. By said term is usually meant a system, which in an emergency braking situation, given substantially the same actuating force, may provide the driver with increased braking power. Systems of said type were developed because investigations revealed that the majority of vehicle users in an emergency braking situation do not press as hard on the brake pedal as is necessary to achieve the maximum braking power. The stopping distance of the vehicle is therefore longer than it need be. Systems of said type already in production use an electromagnetically actuable brake booster combined with a device capable of determining the actuating speed of the brake pedal. If said device detects an actuating speed above a defined threshold value, it is assumed that an emergency braking situation exists and the brake booster is set by means of the electromagnetic actuating device to maximum output, i.e. provides its maximum boosting power.
Brake boosters with an electromagnetic actuating facility are however too expensive for motor vehcles in the low- to medium-price category, which is why solutions have been developed which provide a brake assist function without an electromagnetically actuable control valve. The brake assist function in said solutions is realized in that, after a defined cutxe2x80x94in threshold is exceeded, an armature, which is rigidly coupled to a valve seat controlling the supply of atmospheric pressuire into the brake booster (atmospheric seal seat), is pulled into abutment with a permanent magnet and then holds the said valve seat open until the driver by means of a fixed return stroke of the brake pedal detaches the armature from the permanent magnet.
With said solutions, relatively loud and hence intrusive noises may occur when the brake assist function cuts out, i.e. when the armature pulls away from the permanent magnet. Said noises may occur because the armature, after detaching from the permanent magnet, returns almost immediately into its starting position and therefore, on the one hand, closes the atmospheric seal seat and, on the other hand, opens the connection of the working chamber to the vacuum chamber, with the result that the power assistance generated by the brake booster ceases almost instantaneously. All of the forces, which a master cylinder connected to the brake booster exerts because of the hydraulic pressure generated therein, therefore react fully upon the input element of the brake booster and upon components workingly connected to the input element. Said hydraulic reaction forces are added to the spring forces, which reset the actuating components of the brake booster, resulting in a hard impact of said parts against the corresponding end stops, which causes the said undesirable noises.
The object of the invention is to provide a vacuum brake booster of the described type, i.e. having a brake assist function capable of being cut in and out without electromagnetic assistance, in which said undesirable impact noises no longer occur or at any rate are no longer an intrusive phenomenon.
Said object is achieved according to the invention by a vacuum brake booster having the features indicated in claim 1.
In a vacuum brake booster of the required type a permanent magnet and an armature cooperating therewith are disposed in the housing of the control valve. The armature is rigidly coupled, on the one hand, to the input element of the brake booster and/or of the control valve and, on the other hand, to the first valve seat (atmospheric seal seat). The armature, which is movable to and fro relative to the permanent magnet, is spring-biased counter to actuating direction and in the starting position of the control valve is held at a first distance from the permanent magnet, preferably by means of the spring bias counter to actuating direction. In the course of a movement towards the permanent magnet which occurs upon actuation of the brake booster, the armature may move until it is less than a previously fixed, second distance away from the permanent magnet, which is smaller than the first distance. The armature, once it is less than said distance away, is pulled counter to the spring bias force acting upon the armature and with simultaneous cancellation of its, in actuating direction, rigid coupling to the input element by the permanent magnet into abutment with the permanent magnet. Thus, even when the actuating force introduced into the brake booster does not increase, the first valve seat coupled to the armature remains fully open (brake assist function) so that the brake booster builds up the highest possible pressure difference between its vacuum chamber and its working chamber, i.e. the brake booster provides its maximum boosting power.
According to the invention, the return stroke motion of the input element and/or of control valve components workingly connected to the input element, which follows a detaching of the armature from the permanent magnet, is cushioned before the input element has completed its maximum possible return stroke relative to the control valve housing. Said cushioning or damping of the return stroke motion is achieved in the present invention by the same spring, which biases the armature counter to actuating direction. To said end, said armature-restoring spring is coupled to the input element in such a way that the latter runs onto the spring only after detachment of the armature from the permanent magnet, i.e. after a cut-out of the brake assist function, and indeed before the input element has completed its return stroke. In all other operating states the spring is supported against a housing-fixed component and does not impede the return motion of the input element and/or of components connected to the latter. According to the invention, therefore, the energy introduced into the input element after cut-out of the brake assist function is taken up at least partially by the said spring, thereby preventing a metallically hard impact of the input element and/or of components connected thereto against parts of the control valve housing, which delimit the return stroke.
In a preferred form of construction of the vacuum brake booster according to the invention, a transmission piston, which is rigidly coupled to the input element, is provided in the region of its end remote from the input element with an annular groove, which has a fixed axial extension. Guided in an axially displaceable manner in said annular groove is an annular disk, against which the end of the armature-restoring spring remote from the armature is supported. The annular groove is so positioned and its axial extension is so dimensioned that the annular disk may come into contact with the edge of the annular groove remote from the input element only after detaching of the armature from the permanent magnet, i.e. only after a cut-out of the brake assist function.
The last-mentioned form of construction is preferably developed in such a way that the annular disk, when it is not in contact with the edge of the annular groove remote from the input elementxe2x80x94i.e. in the majority of all of the operating states, is pressed by the armature-restoring spring towards a cover of the control valve housing. In said form of construction, the cover is the above-mentioned housing-fixed component, against which the spring is supported.
For further damping of possible metal impact noises, the annular disk at the side, which after cut-out of the brake assist function comes into contact with the edge of the annular groove, is preferably provided with a shock-absorbing elastomeric layer. With said elastomeric layer the annular disk is supported against the housing-fixed component in the other operating states.
In preferred embodiments of the vacuum brake booster according to the invention, the said second distance is defined by the magnitude of the spring bias acting upon the armature. When the spring bias acting upon the armature is small, this means that the said second distance is relatively large, i.e. the threshold, which has to be exceeded in order to trigger the brake assist function, is relatively low. The reverse is the case when the spring bias acting upon the armature is large. Thus, through suitable selection of the spring bias acting upon the armature and of the said first distance it is possible to define a trigger threshold, which on the one hand avoids unwanted full braking operations but on the other hand may also, if need be, still be reliably overcome by less powerful drivers.
In preferred forms of construction of the vacuum brake booster according to the invention, the first valve seat of the control valve, which controls the ventilation of the working chamber, is rigidly coupled in actuating direction by the armature to the input element. The first valve seat may however also be actuated directly by the input element. In all forms of construction of the vacuum brake booster according to the invention, the first valve seat is preferably formed on an, in particular sleeve-shaped, extension rigidly connected to the armature. In said manner each movement of the armature is transmitted without play to the first valve seat.
When the first valve seat is formed on an extension rigidly connected to the armature, a locking bar rigidly coupled to the input element preferably engages into a recess of the extension, in which the locking bar in the direction of displacement of the input element has a play, which is smaller than the maximum possible actuating stroke of the input element. Thus, the locking bar rigidly coupled to the input element may during the return motion of the input element, i.e. during release of the brake, detach the extension rigidly connected to the armature optionally from the permanent magnet, i.e. cut out the brake assist function. The exact shape of the recess in the extension is in said case unimportant, it merely being crucial that the locking bar or another part rigidly connected to the input element still during the return motion comes into positive engagement with the extension.
In all forms of construction of the vacuum brake booster according to the invention, the input element is preferably spring-biased counter to actuating direction. Upon release of the brake, said spring bias returns the input element into the starting position. In a constructionally advantageous manner said spring bias of the input element in the course of the latter""s return into the starting position is also utilized to detach the armature from the permanent magnet, e.g. by means of the previously mentioned locking bar, which engages into a recess of the extension coupled to the armature.
To reduce or eliminate impact noises which may occur as a result of the return stroke motion of the armature which follows a detaching of the armature from the permanent magnet, in all forms of construction of the vacuum brake booster according to the invention said return stroke motion is preferably damped by means of an elastomeric element, which is disposed between the armature and the stop face, towards which the armature moves during its return stroke motion. In a simple yet effective form of construction, the elastomeric element is an O-ring.
In preferred embodiments of the vacuum brake booster according to the invention, the permanent magnet is guided in a displaceable manner in the control valve housing and is preferably moreover counter to actuating direction spring-biased towards a stop. Such a form of construction has the advantage that, when the armature in the course of a rapid actuation of the brake booster has moved to a distance less than the previously defined, second distance away from the permanent magnet and has consequently been pulled into abutment with the permanent magnet and the actuating force exerted upon the input element is further increased, instead of said actuating force being transmitted from the input element via the unit comprising armature and permanent magnet to the control valve housing and from there to a master cylinder disposed downstream of the vacuum brake booster, an increased actuating force exerted upon the input element is transmitted directly from the input element to the downstream master cylinder. The magnetic device, in particular the armature, the permanent magnet and components receiving the latter, may therefore be of a less stable design. If for all conceivable actuating situations the coming of the magnetic device into a load-transmitting state is to be avoided, then the permanent magnet has to be displaceable in actuating direction relative to the control valve housing by an amount which is greater than the maximum possible actuating stroke of the input element minus the said first distance.
In order, in the event that a stop is provided for the permanent magnet, to prevent manufacturing tolerances from leading to a skew position of the contact surface of the permanent magnet relative to the contact surface of the armature, which in turn may lead to a canting and hence possibly to a jamming of the armature and/or of the permanent magnet, in preferred embodiments the stop for the permanent magnet and/or the contact surface, which is formed on the permanent magnet (or the mounting of the latter) and cooperates with the previously mentioned stop, is of a crowned configuration. Furthermore, there is a radial play between the permanent magnet and/or its mounting and the control valve housing. In said manner, the permanent magnet may always be aligned correctly relative to the armature. Alternatively, the armature itself may be refined in a way that allows a specific swivelling of its contact surface to compensate the said skew position.
Although the permanent magnet may be spring-biased in any manner towards the stop, an, as a whole, annular spring element comprising a plurality of spring ring segments is preferentially used to bias the permanent magnet counter to actuating direction. Such a spring element has a very short axial length and therefore reduces the overall length of the control valve equipped with a magnetic device according to the invention.
According to a preferred development of the vacuum brake booster according to the invention with a displaceable permanent magnet, the armature comprises a base and a magnetic plate, which is connected to the latter and faces the permanent magnet. Thus, only the magnetic plate need be made of a material which is attracted by the permanent magnet, while the base of the armature may be made of non-magnetic material, e.g. a plastics material. If the armature is hollow-cylindrical, then the magnetic plate is preferably annular.